LPDA - shorted or not?

[Apache_UK]

At work we have a couple of 200MHz-1GHz LPDA measurement antennas. One made by Chase Antennas, the other by Eaton. The Chase one has booms which are isolated from each other along their length, the Eaton one has a DC short at the LF (non-feed) end.

Both perform similarly though have slightly different correction factors as you'd expect from different manufacturers.

Why?

I know a short circuit at DC aint necessarily an RF short, but wonder why two antennas which are identical in dimensions and design would have such a fundamental difference such as one being a short circuit at DC, whilst the other is open circuit at DC.

 

[Apache_UK]

In the absence of any explanation on the internet, I am going with open circuit at DC for the UHF Rx one I am building.

The impedance is controlled by the gap between the booms, and I can check I am in the ballpark with our impedance meter in work.

Would be interested in the rationale behind it though if anyone knows.

 

[prcguy]

I believe the short improves the F/B ratio at the lowest frequencies and also helps keep the antenna from becoming a random pickup antenna at frequencies much lower than designed for.
prcguy

 

[Apache_UK]

That makes sense. A kind of belt and braces approach to curtailing the LF response.

Seems odd that they would do it on an EMC measurement antenna, which would be used for a tightly defined frequency span anyway, with receivers that are not likely to struggle with out of band signals.

Interestingly, I noticed when trialling my prototype antenna, that when I shorted the booms as per the Eaton antenna, I struggled to pick up the stuff I normally hear around 40MHz, but when I operated it as open circuit, I did hear them, even though it should be well out of band.